This application is based on and incorporates herein by reference Japanese patent application No. 2000-110978 filed Apr. 12, 2000.
This invention relates to an electrical load driving device having an output transistor which passes a driving current through an electrical load in response to a driving signal from outside. Particularly, this invention relates to an electrical load driving device whose output mode can be switched between a high-side output mode and a low-side output mode.
A vehicle electronic control unit such as an engine control unit or a transmission control unit mounted in an automobile controls respective control objects by driving various electrical loads such as relays and solenoids in accordance with the running state of the vehicle. To each electrical load, depending on the type and the purpose of the load, the electronic control unit uses one of two driving modes to supply a driving current, that is, a high-side driving mode and a low-side driving mode.
In conventional vehicle electronic control units, only one driving circuit (high-side driving circuit or low-side driving circuit) compatible with either one of the high-side and the low-side driving modes is provided on a circuit board. Consequently, when the driving mode of the electrical load is changed, large changes in hardware is necessitated.
To overcome this, U.S. Pat No. 5,828,247 proposes an electrical load driving device which is capable of switching its output mode between the high-side output mode and the low-side output mode. With this kind of electrical load driving device, output modes can be set freely to either the high-side output mode or the low-side output mode, and changes in the driving modes of electrical loads can be accommodated flexibly.
Generally, in a driving device provided in an electronic control unit for supplying a driving current to an electrical load, if the electrical load becomes shorted and an over-current passes through an output transistor of the driving device, it is necessary for a failsafe measure to be taken such as detecting this abnormality and forcibly turning off the output transistor.
In an automotive vehicle, a storage battery is used as a load power supply source of the driving current to the load. The voltage of the battery fluctuates. Consequently, an over-current detecting circuit which can detect that the current flowing through the output transistor becomes excessive even if the voltage of the load power supply fluctuates is necessary.
Thus, to realize a practical electrical load driving device which switches an output mode between the high-side output mode and the low-side output mode, the over-current detecting circuit is required to detect correctly that the current flowing through an output transistor has become excessive whether the output mode is set to the high-side output mode or the low-side output mode and even if the voltage of the load power supply fluctuates.
It is therefore an object of the present invention to provide an electrical load driving device which can detect correctly when the output transistor is in an over-current state, irrespective of a high-side or a low-side output mode or voltage fluctuations of a load power supply.
According to the present invention, an electrical load driving device is constructed to detect an over-current state of its output transistor irrespective of its operation mode, that is, in a high-side output mode or a low-side output mode, and irrespective of voltage fluctuations of a load power supply. In this device, when the output transistor is made to function as high-side driving means, a boosted voltage higher than a battery voltage is supplied to the gate of the output transistor in response to a driving signal. When the output transistor is made to function as low-side driving means, the battery voltage VB is supplied to the gate of the output transistor in response to the driving signal. In the high-side output mode, an over-current detection is carried out by a first detection voltage which decreases from the battery voltage in proportion to the current flowing through the output transistor being compared with a first determination voltage also referenced to the battery voltage. In a low-side output mode, the over-current detection is carried out by a second detection voltage which increases from a ground potential in proportion to the current flowing through the output transistor being compared with a second determination voltage also referenced to the ground potential.